In measurements and inspections of a semiconductor device using a scanning electron microscope (SEM), an electron beam is cast on a sample and therefore electrical charge is formed on the device surface. In some cases, electrical charge may be a factor that causes a distortion and contrast abnormality or the like of an image.
PTL 1 proposes a method in which an electron beam is cast on a contact hole to measure an absorption current value flowing through a substrate, thus observing the hole shape.
Also, PTL 2 proposes a method in which electric potential distribution on a sample surface is made uniform by electron beam irradiation, thus aiming to improve reproducibility of an image. For example, it is disclosed that when a sample is irradiated with energy of 1500 eV, a length measurement error of 6 nm is generated by a potential difference of 10 V. Therefore, a method is proposed in which the electric potential on the surface is made constant by a charge control electrode installed on the sample.
PTL 3 and PTL 4 propose methods aiming to alleviate or stabilize electrical charge that is formed when an insulating sample is observed with the use of a SEM. In PTL 3, the electric field intensity on a sample surface or the primary electron beam scanning method at the time of acquiring an image is controlled, thereby alleviating or stabilizing the electrical charge generated by irradiation with the primary electron beam, and thus making it less susceptible to the influence of the dynamic electrical charge generated by irradiation with the primary electron beam. Moreover, in PTL 4, when inspecting the shape of a substrate surface or measuring the length with the use of a SEM, the signal waveform of a back scattered electron corresponding to the number of times of electron beam scanning until the electrical charge on the substrate reaches an equilibrium is measured, thus identifying the edge of the substance in the scanned area. Alternatively, it is proposed that the signal waveform of a back scattered electron corresponding to the number of times of electron beam scanning until the electrical charge on the substrate reaches an equilibrium is measured and compared with the signal waveform of the back scattered electron corresponding to the number of times of scanning of a known substance, thus identifying the unknown substance in the scanned area.
While the methods for restraining electrical charge are proposed as described above, an observation method utilizing electrical charge is also proposed. With the miniaturization of patterns of semiconductor devices, integration to stack up in vertical direction is being advanced. As structures are stacked up, devices having high aspect ratios (pattern height/hole diameter or groove width) such as a deep hole or deep groove are emerging. For inspections of these patterns, dimensional management of the hole bottom and the groove bottom is required as well as dimensional management of the surface.
In observation of a sample with a high aspect ratio, since the amount of secondary electrons detected from the pattern bottom is small, contrast is not generally achieved and observation and measurement is difficult. Therefore, observation utilizing electrical charge is used. In the case of a material (insulator) that is electrically charged on the surface, positive electrical charge is formed on the sample surface by precharging (pre-dosing) of the surface at a low magnification before observation. After that, observation at a high magnification is carried out, and observation is thus carried out by lifting up the secondary electrons on the hole bottom with the positive electrical charge on the surface.
PTL 5 proposes a method in which dimensional measurement at an arbitrary depth is carried out, based on the amount of positive electrical charge formed on the surface. It is considered that the depth where lifting of secondary electrons is possible can be controlled by adjusting the amount of electrical charge on the surface.